In recent years, with computers and information communication equipments developing toward high performance, high function and networking, to transmit and process large-capacity information at high speed, operation signals tend to be high-frequency. Therefore, the material of the circuit substrate material is required to be improved.
In a high-frequency circuit, the transmission loss of electrical signals is expressed by the sum of dielectric loss, conductor loss and radiation loss. The frequency of the electrical signals is higher, the dielectric loss, conductor loss and radiation loss are more. The transmission loss weakens the electrical signals and destroys the reliability of the electrical signals. So, the decreasing of the dielectric loss, conductor loss and radiation loss on a circuit board processing high-frequency signals needs to be inhibited. Dielectric loss is proportional to the product of the square root of the dielectric constant of the insulator forming the circuit, the dielectric loss tangent, and the frequency of the signals used. So, as an insulator, the decreasing of the dielectric loss can be inhibited by choosing an insulating material with low dielectric constant and the dielectric loss tangent.
Among the conventional materials used for a printed circuit substrate, the epoxy resin with excellent adhesion characteristics is widely used. However, the epoxy resin circuit substrate has relatively high dielectric constant and dielectric loss tangent (dielectric constant is more than 4, and dielectric loss tangent is around 0.02), and inadequate high-frequency characteristics, which results in failure in meeting the requirement of high-frequency signals. Polytetrafluoroethylene, as a representative of fluor resins, is always being used as the substrate material for processing high-frequency signals, since it has low dielectric constant and dielectric loss tangent. However, polytetrafluoroethylene is a highly inert thermoplastic material, which is hard to be molded and processed. So, researchers in the field do various researches on insulating materials with low dielectric constant and dielectric loss tangent, which are easy to form varnishes by utilizing organic solvents, are easy to molded and processed, have low curing temperature, are easy to operate, and are not of fluorine. Eliminating or decreasing polarity groups in the molecular structure is good for obtaining materials with low dielectric constant and dielectric loss tangent, and proposals have been provided, such as cured polyolefin, resins of cyanate ester, cured polyphenylene ether, allyl modified polyphenylene ether, vinyl modified polyphenylene ether, polyvinyl benzyl ether resin, divinylbenzene or divinyl naphthyl modified polyether imide, and liquid crystal resins. These resin compositions with low dielectric constant and dielectric loss tangent are designed as thermosetting resins, since they must endure electronic unit manufacturing procedures such as solder reflux and metal wire welding.
In these existing proposals, cured polyolefin resin materials are attracting more attention since they have low dielectric constant and dielectric loss tangent that are comparatively low and close to polytetrafluoroethylene. For a long time, technicians in the field have studied thermosetting polybutadiene or copolymer resin of polybutadiene and styrene with excellent dielectric properties. The results of these studies are cited as follows.
PCT patent application No. WO97/38564 disclosed to use nonpolar styrene-polybutadiene divinyl benzene terpolymer added with magnesium aluminum silicate as filler, and use fiberglass cloth as reinforcing material to make circuit substrate which has excellent dielectric properties but poor heat resistance; the glass transition temperature of the substrate is only about 100° C., and the coefficient of thermal expansion is very big. So it is hard to meet the high temperature (over 240° C.). requirement in the lead free process of manufacturing PCBs.
U.S. Pat. No. 5,571,609 disclosed to use 1,2-polybutadiene resin or polyisoprene with molecular weight being less than 5000, to cooperate with polybutadiene styrene copolymer with high molecular weight, added with many silica as filler, and use fiberglass cloth as reinforcing material to make circuit substrate which has excellent dielectric properties, but in the patent, high molecular weight (Mn is over 50000) component is adopted to improve tacky property of the prepregs, so the processing property during manufacturing prepregs becomes poor. The proportion of rigid structure benzene ring in the resin molecule of the whole resin system is very small, and the cross-linked segments are mostly composed of methylenes with very low rigidity. Thus, the made sheet has poor rigidity and very low bending strength. Besides, the patent adopted much silica as a filler to improve tacky property of the prepregs, and the hardness of silica is very big, so, the drill bit will be worn more during the PCB processing.
U.S. Pat. No. 6,569,943 disclosed to use amine modifier polybutadiene liquid resin with vinyl at the molecule terminal, which is added with monomers of middle or low molecular weight as curing agent and diluent, and use impregnate fiberglass cloth to make circuit substrate. Although the dielectric property is good, for the resin system is liquid at normal temperature, it can not be made into tack-free prepreg. Therefore, in the compression moulding of sheet, it is difficult to adopt common stacked technology of prepreg, which results in difficult process operation.
In the Chinese paten application No. 200910106628.9, allyl novolac of middle or low molecular weight was used as the curing agent to effectively improve the tacky property, heat resistance and mechanical strength of prepregs, and to obtain a comparatively good dielectric property.
The present invention is a further improvement based on the Chinese paten application No. 200910106628.9, by introducing a solid styryl resin of middle or low molecular weight with unsaturated double bonds. Not only the advantages in the Chinese paten application No. 200910106628.9 are obtained, but also the dielectric property is further improved.